In many surveying apparatuses, the modification of oriented optical measurement radiation is required, for example, by aligning components which integrate the beam path or by focusing on a target. In this case, this modification must be performed precisely and, in the case of dynamic applications, also sufficiently rapidly, and as a result large masses to be moved are disadvantageous, for example. In addition, the suitability of the surveying apparatus for use in the field in principle requires robustness of the drive and a low power consumption.
An industrial or geodetic surveying apparatus typically has a vertical axle and a tilting axle, which are mounted and driven. Examples of such surveying apparatuses are total stations, theodolites or tachymeters, which—also together with integrated automatic target acquisition and target tracking devices—are used for multifarious surveying tasks, wherein both data acquisition and a mere examination are suitable, such as for example in structural monitoring. Other measuring apparatuses are, for example, systems such as laser trackers, laser scanners or profilers, which record topographies of surfaces as three-dimensional point clouds during the scanning method.
In solutions used to date, servomotors which represent the drive of the components are frequently used for these purposes. Servomotors with a downstream transmission are complex, expensive, require significant outlay in terms of maintenance and are susceptible to wear. Combinations of electric motors and transmissions are additionally not able to connect high axle speeds that are required for scanning with exact and slow fine-targeting or positioning, since generally the usable speed range of the motors or transmissions is not large enough herefor.
For minimum displacement distances, on the other hand, alternative drive elements can be used, such as for example electrostatic combs. U.S. Pat. No. 7,999,921 and EP 1 314 960 disclose, for example, geodetic measuring apparatuses in which piezoelements are used to move a radiation source within the housing of the measuring apparatus. However, the available movement range is limited by the deformation of the piezoelement. In particular, a continuous and unlimited movement as endless drive, as is required for example for a rotation, is not realizable by a piezoactuator or electrostatic apparatuses. Piezodrives are based on the principle of frictional engagement and are therefore prone to wear. Occasionally, noise may also be produced due to the principle. Piezodrives are moreover not suitable for high speeds. It is often advantageous for the scanning of objects if the measurement beam is moved in an oscillating fashion. This application is not continuously possible on account of wear.
Asynchronous drives may be solutions which are often used in continuous, continual, uniform movements, but cannot be used for positioning tasks in surveying instruments since the static regulation of asynchronous motors is nearly impossible.
Synchronous motors would be suitable in radial construction for surveying apparatuses, but on account of the construction, have cogging torques. With non-energized winding, the magnets of the rotor thus align with respect to the stator lamination, as a result of which the axis moves away from the predetermined position. Permanent regulation or energizing is necessary to keep such drives in position. For surveying apparatuses such as total stations, precise slow movement is necessary to be able to align the telescope with a target. This uniform movement is achievable only with difficulty owing to the cogging torque. It is often desirable to have a free internal diameter of the rotor that is as large as possible in order to use that space for guiding optical beams and/or cables through it. However, in this type of motor, the external diameter also increases strongly due to its function, as a result of which the motor can be completely oversized. In addition, owing to tolerances, synchronous motors in radial construction generate a radial, static force which is inadmissible in precision slide bearings.